1. Field of the Invention
This invention relates to copolymers, and more specifically relates to polyurethaneureas which may be extruded and molded in melt form and a method for their preparation.
2. Background of the Invention
Polyurethane block copolymers possess an outstanding balance of physical and mechanical properties and superior blood compatibility compared to other polymers such as silicone rubber, polyethylene, polyvinyl chloride and perfluorinated polymers. As a result, they have come to the fore as the preferred polymeric biomaterials for fabrication of various medical device components. Some important device applications for polyurethanes include peripheral and central venous catheters, coatings for heart pacemaker leads and the Jarvik heart.
Polyurethanes are synthesized from three basic components, a polyisocyanate, a macroglycol and an extender, usually a low molecular weight diol, diamine, aminoalcohol or water. If the extender is a diol, the polyurethane consists entirely of urethane linkages. If the extender is water, aminoalcohol or a diamine, both urethane and urea linkages are present and the polyurethane is more accurately and conventionally termed a polyurethaneurea. In this disclosure, polyurethaneurea will hereinafter be abbreviated as PUU.
Polyurethanes develop microdomains conventionally termed hard segments and soft segments, and as a result are often referred to as segmented polyurethanes. The hard segments form by localization of the portions of the polymer molecules which include the isocyanate and extender components and are generally of high crystallinity. The soft segments form from the polyether glycol portions of the polymer chains and generally are either noncrystalline or of low crystallinity. One of the factors which determines the properties of the copolymer is the ratio of hard and soft segments.
Exemplary of important diol extended polyurethanes are: Vialon.RTM. (Becton Dickinson Polymer Research), Pellethane.RTM. (Dow Chemical Co.,) and Tecoflex.RTM. (Thermedics Inc.). These products typically have good blood compatibility, but, with the exception of Vialon.RTM., generally require processing additives such as antioxidants and detackifiers, a potential disadvantage for use in biomedical articles. They are, however, thermoplastic and therefore may be melt extruded and injection molded.
Diol extended thermoplastic polyurethanes are conventionally manufactured by operationally simple and economical bulk or one shot polymerization processes wherein all the ingredients are combined, mixed and reacted. PUUs, although commercially prepared by a two step procedure, described below, have also been prepared by a one shot continuous process using a catalyst (U.S. Pat. No. 3,642,964 to Rausch et al.). The catalyst, because is is generally inherently toxic, cannot be present in PUUs to be fabricated into biomedical articles.
The conventional two step preparation of PUUs is generally carried out by reacting the isocyanate and macroglycol in a solvent to give a prepolymer followed by chain extension with the diamine or aminoalcohol. Exemplary of the two step procedure is the disclosure of Gilding et al. in U.S. Pat. No. 4,062,834.
Several disadvantages are encountered in the two-step process. First, the process generally requires a solvent, usually toxic dimethylacetamide (DMAC). Second, as pointed out by Ward et al. (Polyurethanes in Biomedical Engineering, H. Planck, G. Egbers and I. Syre, ed., Elsevier Science Publishers B.V., Amsterdam, 1984) even reagent grade solvents contain enough water as an impurity to hydrolyze a significant portion of the isocyanate groups to amine groups which react with other isocyanate residues to form urea linkages of different structures than those obtained from the diamine extender. The resulting mixed hard segments complicate the structure of the polymers and increase the likelihood of batch to batch variations in properties.
A PUU well-known as an industrial fiber (Lycra.RTM. DuPont de Nemours and Co.) has been extensively studied under the trade name Biomer.RTM. (Ethicon Corp.) for fabrication of various biomedical devices. A review of these studies and the many salubrious properties of PUU has been presented by Phillips et al., The Use of Segmented Polyurethane In Ventricular Assist Devices and Artificial Hearts, in Synthetic Biomedical Polymers, M. Szycher and W. J. Robinson, ed. Technomic Publishing Co., Inc., Westport, Conn., 1980, page 39. However, as stated by Phillips et al., Biomer.RTM. presents some fabrication difficulties that limit production techniques. Biomer.RTM. has a melt temperature higher than the decomposition temperature of the urethane functionality and therefore can be spun or cast only from solution, i.e., it cannot be melt extruded or injection molded. Severe limitations are thereby imposed on its fabrication latitude. Further, it is essentially insoluble in all solvents except DMAC which of course must be completely removed if the product is to be used in a biomedical article.
Taller et al., in Research Disclosure No. 12,823, December 1974, and Short et al. in U.S. Pat. No. 4,522,986, disclose PUU compositions prepared by the two step prepolymer technique from a diisocyanate, a polyol and monoethanolamine as extender.
Ward et al. (supra) discloses a new PUU formulation for biomedical use consisting of a blend of PUU and an additive surfactant polymer.
There is a need for a bulk polymerization method to prepare melt processable PUU having the desirable properties of both diol extended and diamine extended PUU. The present invention fulfills this need.